1. Field of the Invention
The present invention relates to a valve timing control system, which controls opening and closing timing (hereinafter, simply referred to as valve timing) of at least one of an intake valve and an exhaust valve of an internal combustion engine.
2. Description of Related Art
In a previously proposed valve timing control system, a rotational phase of a driven shaft relative to a driving shaft is controlled by a fluid pressure of working fluid, which is supplied to retarding chambers and advancing chambers, to control valve timing of at least one of an intake valve and an exhaust valve (see, for example, Japanese Patent No. 2998565). The supplying of the working fluid to the retarding chambers and the advancing chambers and draining of the working fluid from the retarding chambers and the advancing chambers are controlled by a fluid control valve, which is formed as, for example, a known solenoid spool valve.
However, an opening area of the fluid control valve is smaller than a fluid passage of other devices other than the fluid control valve. Thus, when a viscosity of the working fluid, such as hydraulic oil, is increased under the low temperature condition, a flow quantity of the working fluid, which is supplied from the fluid control valve to the retarding chambers and the advancing chambers, is reduced in comparison to the high temperature condition. Thus, the time, which is required to fill the hydraulic oil into the retarding chambers or the advancing chambers to execute the phase control, is disadvantageously increased, so that the response in the phase control is reduced, i.e., impaired. When the response in the phase control is reduced, the timing for controlling the valve timing is deviated.